Posted by: Li Ling Young | February 5, 2017

Dust Blows

I don’t check our whole-home energy monitoring system several-times-a-day like I used to, so it was pure coincidence that I happened to catch this when I was computering the other day.


A normal water heater cycle is shown in dark blue on the left. A freakishly long water heater cycle is shown in dark blue on the right. Spikes in the water heater power is likely the electric resistance element coming on to help the heat pump.

There’s a lot going on in this graph, but focus on the dark blue shapes.  That shows when the water heater came on, how long it stayed on and how much power the water heater is drawing.  The water heater cycle that starts around 10:30 pm goes on for 7.5 hours!!  What? Too much.  The cycle on the left is more normal for this time of year.  In the summer when the basement is warmer the water heater cycle is a little shorter.  But 7.5 hours to reheat itself after we… what? washed dishes?

Actually, I didn’t really question it much when I first saw that.  But a couple of days later I happened to glance at the water heater and saw this


Dust accumulating in the heat pump water heater.

That’s dust on the fins in the heat pump that makes our hot water.  For a primer on heat pumps, see this post.  The fins are where energy in the air is transferred to the refrigerant.  A lot of air has to move through the fins to deliver enough energy to make hot water. There’s just a teensy space between the fins, and as we are discovering dust can get caught on the way through.  What’s the problem?  As the space through the fins gets clogged up the fan that blows air through there has to work longer to deliver enough energy to the heat pump.  At this point, as I’m standing in the basement contemplating the dusty water heater I remember the thing I saw on our whole-home energy monitor: a long, long water heater cycle.  This dust is doubling the amount of energy our water heater uses.

Nik got on a stool and vacuumed the fins.  If you try this at your house, be careful!  The fins are easily damaged.  Use the brushy attachment on your vacuum.  Here’s a shot after Nik’s work.


The water heater after a vacuuming-out. Cleaner fins equals better efficiency.

What you can’t see is the lower part of the fin-assembly, hidden behind the black box.  That part is still gunked up and there is absolutely no way to get at it with the vacuum.  I asked the Stiebel Eltron manufacturer’s representative about that a couple of days later and he said, “It’s supposed to be self-cleaning.”  I pointed out that all other heat pump water heaters have a filter over the heat pump compartment.  A real let-down from a company from whom I expect top-notch engineering. Later an old timer-heating installer told me to use compressed air to blow it out.

Since the cleaning we’ve had a few more very long water heater cycles.  Guess the dirty fins weren’t the whole story.  Or maybe it’ll right itself once we blow out the rest of the fins.

Today I was in a funk and was casting about for some comfort cleaning.  Nik said he hadn’t cleaned the filters on the mini-split heat pump lately and offered to show me how he does it.  Just like the heat pump water heater, the mini-split blows air through a fin assembly to move air in and out of the refrigerant.  Unlike the the water heater, filters protect fins from getting clogged up with dust.  Nik cleans them every month in the winter.


Nik cleans our mini-split filters because it keeps the mini-split working well, and all that dust is icky: get it out!


Oooh, yeah. Filters need attention. I guess I should be glad the mini-split has these filters. It keeps our air cleaner!

Heat pumps need a little care.  Not much, really.  The consequences of neglecting them is lower performance and higher energy use.  We want the opposite, so we’re happy to keep up with whatever the heat pumps need.

Posted by: Li Ling Young | January 29, 2017

This Kind of Wall

Sometimes you look back in history and say to yourself, if I was there I would have… Sheltered Jews, Protected Japanese neighbors’ property, Befriended the only black children in the school…  This history, being made right now, is our chance to be on the right side of justice, to be the citizens we will wish we had been and to make our children proud when we tell them what we did when democratic institutions were burning.

In 2017 this is the kind of wall I want.  It’s cheaper than a border wall.  It fights climate change.  It pays for itself.  And it’s made in the USA!

After several years of planning we are adding insulation to the outside of our walls.  Since our house, like most houses in the US, has 4″ walls, there isn’t much room to work with when it comes to insulating.  For more than 4″ of insulation we need to take space on the inside of the wall or on the outside.  Obvious disadvantages of adding insulation to the inside: very expensive to restore the finishes; we’re living here and don’t want to deal with construction mess/displacement; makes the rooms smaller.  Obvious advantage of adding insulation to the outside: chance to replace the 60+ year-old siding that was damaged when we had the walls densepacked.  Other pros and cons are not listed here, but suffice it to say that these were the compelling factors

Why even bother?  As Nik has pointed out more than once, we’re already providing all our household energy with our solar system (and about a cord of wood a year – which you can count  under the zero energy column or not per your attitude toward the renewability and sustainability of burning wood).  If we “save” energy by becoming more efficient, we’re not going to save any money, and we’re not even going to reduce pollution/forestall climate change.  What are we gaining by making our home better insulated?  Comfort! What I have pointed out more than once is this home is the worst kind of zero energy home: uncomfortable.  Because heat is provided only in the dining room, parts of the house far from the mini-split or woodstove are cooler than the core of the house.  The colder the weather, the cooler the bedrooms until it gets fully cold in our bedroom and bathroom.  I’ve seen it below 50 deg F in our bathroom: time to shower at the gym.

The design of the new wall is, in my opinion, an improvement on every thickened-wall strategy I’ve seen, even the cherished Larson Truss.  Nonetheless, it’s not really what I wanted.  One of my objectives was to design a foam-free wall because spray foam is dangerous for the installer and is made with chemicals that contribute to climate change.  My wall has a little bit of foam in it.  I also wanted to leave the original siding in place so as to not disturb the lead paint.  Got talked out of that because it makes the construction easier to have the old siding out of the way.  Here’s what makes it a superior way to add insulation to the walls in an existing home:

  • minimal foam
  • faster to assemble than Larson Truss
  • good structural design
  • relatively few expensive fasteners
  • nailbase
  • easy to hang windows and trim

Step One – Our awesome builder, Ed, removed all the siding on two walls by himself in two days (including cleanup and disposal).  He also framed in the opening for the larger windows that will go on the south side.  We were able to check in on the densepack job that was done four years ago.  From what we saw, good fill but not that dense.  You can also see there was a narrow bay that was missed entirely.


The windows on the south side of the house will be enlarged to let the bedrooms load up on solar heat in the winter and to provide egress. Sheathing removed shows the cellulose that was added to the walls a few years ago and the fiberglass batt that originally insulated the house.

Step Two – Ledgers are mounted on the wall.  An inch-and-a-half of polystyrene foam board is behind the ledger to provide a continuous insulation layer and make even more space for insulation.  Originally I asked for a 2X8 to be added to the wall, but with this 1.5″ of foam board behind the ledgers only a 2X6 is needed – saving trees!  The ledgers are attached with very long screws through to the structural 2X4 in the original wall.  These are the only long screws required in the whole assembly.

After the siding is removed ledgers are mounted to wall with 1.5

After the siding is removed ledgers are mounted to wall with 1.5″ of rigid foam board as a spacer.

Step Three – New studs are notched to hang on the ledgers.  This makes a super strong wall with minimal fasteners, and no time spent assembling trusses.  On the gable end a small shed roof is constructed to cap the new wall.  One thing I really like about this little roof is it keeps water off the wall, which will help the paint last longer and make sure the windows never leak.


The walls have been thickened out with a 2X6 spaced off the wall by 1.5″. The two eaves have been connected with a tiny shed roof to cover the thickened wall. Here you can see the tar paper that was behind the siding, but this paper will be removed so the spray foam can adhere directly to the sheathing.

Step Four – At this point work on the wall is stopped while some excavation happens.  The basement under this part of the house is finished with nice knotty pine boards that I don’t want to remove, so we had the foundation wall dug out and insulated on the outside (4″ of extruded polystyrene – R-20).  Ed designed the whole thing so the foundation insulation would butt up- and get glued to the foam board behind the lower ledger.  We’re also enlarging two windows in the basement for egress and to improve daylighting down there.  Ed cut out the foundation wall and framed in the new window to be flush with the outside of the foundation insulation.


Foundation insulation on the outside keeps the basement walls nice and warm, avoiding summertime condensation. The pink insulation aligns nicely with the blue insulation peeking out behind the lower ledger.

Step Five – One inch of high density spray foam is applied to the original sheathing of the house: behind studs and over ledgers.  This air seals the wall and provides a vapor diffusion retarder.  To keep moisture inside the house from working its way out to the new sheathing this layer should be 2″.  But I wanted to keep foam to a minimum and I’m pretty sure there’s oil paint on the inside of the walls, and that will do a pretty good job of keeping moisture out of the walls.  By all rights this should be attended to seriously, but this is my house and I’m willing to take the risk because I think it’s quite low given the whole wall assembly.  I also know that I will keep humidity inside the house under control. There are a lot of things that contribute to whether or not a wall manages moisture well and I feel good about this design.  In other circumstances, 2″ of high density spray foam is smart.


Spray foam on walls gets between the studs and the old sheathing, making sure the insulation is not interrupted anywhere. At this point in the construction the house has suddenly gotten much more air tight. It’s been warm, but I’m expecting to really tell the difference during the next cold snap.

The original sheathing on the house is 3/4″ planks.  Gaps have opened up between the planks as they have shrunk, so the spray foam is making a big difference in the air tightness of the wall.  There’s a lot of chatter about where this air tight layer should be in a thick wall.  Having it in the middle, with insulation to the inside and the outside, is a little unusual, but I think it’s the best place.  All the insulation to the outside of the original plank sheathing keeps it warm so it won’t support condensation in cold weather. Applying an air barrier and vapor diffusion retarder at that surface keeps moisture from getting into the outer wall and that avoids condensation on the outer sheathing.  I had wanted the air barrier and vapor diffusion retarder to be a peel-and-stick membrane (like Grace Ice and Water Shield), but because of the irregularity of the plank sheathing my builder wasn’t comfortable with that.  And that’s how I ended up with foam in the wall.

Step Six – Time to make a mess inside.  Windows are removed from the original wall and replaced to the outside of the new wall.  The windows were new about 15 years ago and they’re pretty nice.  We’re keeping them except in the places where the windows are being enlarged for more sunlight and egress.  OSB sheathing is installed all over the wall.  The space behind the OSB is 7″ (5.5″ for the “studs” and 1.5″ for the foam board; the ledger is notched into the “studs” so it doesn’t add any thickness).  The remaining space is filled with cellulose, so total insulation is 1″ spray foam + 6″ cellulose = R-28.

All the rest is siding and trim.  Lots of folks want to know how much this is costing, and is it “worth” it.  I won’t know total cost for a while, but I’m guessing about $30,000.  We could have gotten another heat pump for less than $6000.  But we wouldn’t have new siding out of that, or egress windows in the bedrooms, or a warmer basement…  How much is comfort worth?  And how comfortable will the bedrooms really be?  Time will tell. Even Nik, with his preference for mechanical solutions, admits that he can feel a difference in the bedroom, and I think I detect a note of appreciation.  We’re only insulating two walls: the ones furthest from the mini-split and woodstove.  That keeps cost down and focuses our efforts where we need to improve comfort.  So, worth it..?  I know some would say no.  For me, there are benefits that can’t be monetized, and obviously my answer is yes to the wall.

Posted by: Li Ling Young | March 20, 2016

Your House is a Battery

(originally posted in the Vermont Zero Energy Home Pilot blog, where I write as a building science professional, rather than as a homeowner)

We used to get this question a lot, “Do I really save energy by letting my house cool off when I’m at work, since the heating system has to work so hard to warm the house back up later?”  Vermonters are frugal and it’s common to use a temperature “setback”: allowing the house to be cooler than is comfortable when no one is home, or when everyone is asleep. But, there is skepticism that it doesn’t actually save energy since warming the house back up (surely) uses so much energy.

My stock answer to this question is, Yes.  Courtesy of my colleague, Paul Scheckel, an analogy: When you want to make tea, do you keep the kettle hot all day, only to pour your orange pekoe at 4pm?  No, you turn the kettle on when you want the water hot.  The moral: there is no need to keep your house warm all the time if you’re only going to enjoy that warmth for a few hours in the morning and a few at night.  Caution, I’m going to contradict this later on: DON’T TAKE MY WORDS OUT OF CONTEXT.

Sometimes my questioner shoots back, “But my house doesn’t ever get down to 62 (or whatever the setback temperature is) degrees.”  The purpose of the setback isn’t to make the house colder, it’s to prevent the thermostat turning on the heating system (as frequently) during the setback period.

But why didn’t the house cool off?  Because it’s insulated.  Teakettles aren’t insulated.  The better insulated and air sealed a house is, the more slowly it cools off.  Very air tight and well-insulated homes cool off so slowly that they only call for heat infrequently, whether or not the house is in setback.  Efficient homes save less from thermostat setback than poorly-insulated and leaky homes.   Older, poorly-insulated homes can save quite a  bit of energy by using thermostat setbacks, and that’s probably why this strategy has taken hold in the Vermont imagination as a way to be frugal.

The reason heating the house back up after setback doesn’t incur an energy penalty is that most heating systems operate at the same efficiency no matter how much heating they have to do.  In fact some heating systems, particularly oil -burners, are a little more efficient when they run for a long time, as they would when warming the house back up.

Now comes a new type of heating system – the cold climate air source heat pump.  (Also occasionally known as: ductless heat pump, mini-split, heat pump, air source heat pump…)  The defining feature of a cold climate heat pump is that it’s variable speed; it doesn’t operate only in on-mode or off-mode, it can put out a little bit of heat or a lot of heat.  Cold climate air source heat pumps are most efficient when they are not working very hard: very much like a car rolling along at a constant speed.

Also unique to cold climate heat pumps is that they make less heat as the temperature outside drops.  So, if you combine the lower efficiency and lower output at low temperature you can see that you don’t want to ask your heat pump to do a lot of hard work when it’s very cold outside.

heat pump capacity temp

As the outdoor temperature (horizontal axis) drops, an air-source heat pump loses capacity to make heat.  This model only makes about 70% of it’s rated capacity (vertical axis) at -13 deg F.  As a side note, these heat pumps have been observed making plenty of heat at -24 deg F.  Cold climate heat pumps work great, but you have to size them properly for your climate and your house.

So how should one operate a house heated with a cold climate heat pump?  Do not set the temperature back, ever.  By keeping the house at the same temperature it’s like driving a car at a constant speed on a flat road: you only have to put a little energy in to keep going.  On the other hand, recovering from a setback period (changing the temperature of the house) is like accelerating a car uphill: you’re asking the car to work hard, it does so less efficiently AND your car has less power the steeper the hill is.  The car won’t be able to do what you’re asking , and it will use a lot of gas trying.  Or, to shed the analogy, your house won’t recover from the setback very quickly, and it will do so at the lowest efficiency.  So don’t accelerate and don’t drive uphill.

What about the teakettle?  Isn’t it wasteful to keep the house toasty warm when no one is home?   Remember, well-insulated and air-sealed homes don’t gain much from letting the house cool off.  In fact, given the unique nature of a cold climate heat pump it might actually make the most sense to heat the house up when no one is home.  If the warmest part of the day is at 1pm, when everyone is at school or the office, it might be best to ask your heat pump to get the house extra warm at that time, when it can do so at a higher efficiency, rather than expecting the heat pump to do a lot of heating work at 5:00 when everyone is home but the temperature has dropped 10 or 20 degrees.  But ONLY if the house is well-insulated and air-sealed so it can hold onto that heat energy long enough that you get to enjoy it later.

This is how a well-insulated house is like a battery.  You can fill it up with heat energy at the best time, and use the heat energy later when you want to be comfortable at home.  It only works if your house can hold the heat, and it’s only beneficial if you’re using a cold climate heat pump.

I have two purposes in explaining all this:

  1. Point out how important both building improvements and advanced heating systems are; and how they work together to make a whole greater than the sum of the parts
  2. Make a compelling case for why one operates a heat pump-heated home differently from a fuel-heated home.

Here’s my best advice on how to operate your heat pump for best efficiency and comfort

  • Set the temperature you want the house to be and leave the thermostat there all season (winter operation; in the summer just turn the AC on when you want it)
  • Leave all the interior doors open when no one is home (good idea all the time, but desire for privacy may trump this)
  • In very cold weather you can get more heat out of the heat pump by putting it in high fan speed.  It’s noisy, so I advise doing this when you’re asleep or when no one is home (again, using the battery-function of a well-insulated home)
  • Clean the filters regularly
  • Make improvements to the insulation and air tightness of your home!

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